How and why does 5% Non-fat dry milk block non-specific binding sites on proteins in a western blot?

Answer 1

Transfer membranes used in western blotting have very high affinities for proteins. The protein contianed in the milk will saturate and/or block access to the remaining protein binding areas of the membrane.

Answer 2

It just provides so much protein that any thing on the membrane that can bind protein binds the proteins in milk. The antibody, then, can only bind to the antigenic determinants on the molecule it is directed against, and won't bind to the membrane non-specifically.